Info for GMC but may be relevant to Chevy trucks:
http://www.gmpartswiki.com/getpage?pageid=24549

You still haven't identified which year transmission you have (read my previous post).
SM-319 is the 4 bolt, SM-326 is the 7 bolt
You still haven't identified how many teeth are on the gear that meshes with the speedometer drive gear.
While inside, do a bit of measurement and get a rough idea on stem length.
The vendors who sell these driver gears should be able to tell you the stem length of what they sell. Give then a call.

I'm bowing out on this topic. Please post your solution when you find it. Good luck.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."